Various types of speed signal generating circuits are known, and, especially circuits of such type which are coupled to inductive pick-ups and which include signal processing functions to provide square-wave output signals or output pulses in dependence on the received alternating, usually sinusoidal input signals. The signals are passed through an amplifier which, as usually provided, has hysteresis. The hysteresis characteristic is useful to suppress disturbance signals which can appear on connecting lines from the inductive pick-up. Hysteresis, as used herein, is understood to mean that the amplifier provides an output signal when a first predetermined threshold level is exceeded, and continues to provide the output signal until the input signal drops below a second threshold level which is less than the first predetermined threshold level.
The system can be used with anti-brake lock systems, anti-slip regulating systems or the like, and, typically, can be used in connection with many applications in which the rotary element is the wheel of a vehicle, for example and typically an automotive vehicle. When the speed of the wheel is low, the amplitude of the resulting output signal will be small. It may, thus, occur that the first threshold limit of the hysteresis is exceeded only sporadically. An anti-brake lock system (ABS), connected to receive the output signals from the hysteresis-type amplifier, may thus provide erroneous output signals. The input signals to the ABS controller may cease, since the level of the input signal applied to the amplifier is insufficient to exceed the first threshold level. Lack of information representative of wheel speed will cause the controller of the ABS to logically assume that the speed of the wheel has decreased further, and, thus, control a brake to lesser braking value or braking effort. Consequently, a vehicle so equipped will not be braked to its maximum efficient value, since the controller itself, based on electronically erroneous input information, has assumed that a wheel may have been blocked while, actually, it is still rotating.
Various solutions to the problem have been attempted; most of them require a large number of circuit components and sophisticated networks.